Spectroscopy: Current Grism Set

Long-slit spectroscopy

UIST has two 9-slot grism wheels which contain the polarimetry prisms and the grisms for spectroscopy. 1, 2, 4, 5 and 7-pixel slits are available for use with each grisms (the exceptions being the IJ and JH grisms, which have their own 2 and 4-pixel slits - see previous page). The table below lists the grisms currently available.

Note that with the wider slits the spectral resolution is reduced, though with the narrower slits it is usually improved, roughly by the ratio of the slit widths. Unfortunately this is not the case with the IJ and JH grisms, where only a ~30% improvement in spectral resolution is seen when the 2-pixel slit is used instead of the 4-pixel slit.

The spectral resolution with the IFU is roughly equivalent to a 2-pixel slit, so is double that given below.

Click on the name of a grism below... to see the relative transmission across the passband of that grism. Each plot shows a spectrum of a bright standard star. The spectrum has been "normalized" via division of an appropriate black-body function, thus giving the transmission (though note that the absolute scale on the y-axis is arbitrary). Absorption due to the atmosphere plus the telescope and instrument optics (especially the grism and spectral blocking filter) all contribute to the overall shape of each plot. Photospheric absorption lines associated with the standard have not been removed. Note that UIST's throughput drops quite considerably towards the I-band, and that the long-wavelength end of the JH grism is suppressed by the blocking filter (see below).

A comparison of IJ, JH and HK spectra obtained through a 4-pix slit of HIP 87895 (G2V, V=6.3), after division by an A0V star (HIP 85382, 5.9 mag) for telluric correction, is given here (wavelength scales are approximate).

Note also that in RAW data frames the wavelength increases to the LEFT ; the pipeline software will, however, display the wavelength increasing to the right (further details are given in the pages on Data format).

Low versus Moderate-Resolution Grisms

Should I use a moderate-resolution or a lower-resolution grism? The answer depends on your needs. The relative transmissions are similar. However, with most of the higher-resolution grisms background-limited performance is essentially impossible, so (read)noise on the array can be a dominating factor. The higher-resolution grisms work well if one is trying to detect line emission superimposed on continuum emission, and obviously they offer higher spectral resolution.

Figure 1: Comparison of short-K (left) and HK (right) spectral images of the same target, using the same (60 second) integration time. The data are flat-fielded and sky-subtracted, though not corrected for telluric absorption (i.e. no division by a standard star). In addition to continuum from the star, faint H2 emission at 2.122 microns is detected along the slit.

To compare the performance of the HK and short-K grisms, a young star with a line-emission jet was observed with both grisms, using the 4-pixel slit and 60 sec exposures in both cases. In both datasets, continuum from the star and weak line emission from the jet were detected (Fig.1). However, in extracted spectra from the jet (Fig.2 - left) a low-frequency "ripple" is evident in the short-K data which isn't apparent in the HK data: this is produced by a "chevron" readnoise pattern across the array, which though variable can dominate the noise at very low flux levels.

Figure 2: Comparison of short-K (red) and HK (yellow) spectra extracted from along the jet (left) and from the star itself (right). Rows 795-805 were extracted and coadded for the left-hand spectra; optimal extraction was used for the right-hand spectra (for display purposes 3000 counts were added to the short-K spectrum at right). The spectra have not been smoothed.

The short-K spectrum extracted from the star fairs much better (Fig.2 - right): in this case the main source of noise is the continuum from the star. At the higher spectral resolution of the short-K grism, the (spectrally-unresolved) line-emission towards the star is more prominent in the short-K data than it is in the HK spectrum.